1. Field of the Invention:
The present invention relates to a desulfurization method and a desulfurization system to remove sulfur from hydrogen sulfide-containing gas by use of cells at a geothermal power plant.
2. Description of the Prior Art:
The hydrogen sulfide-containing gas generated at a geothermal power plant has so far been desulfurized by a plant using ferric sulfate. In said desulfurization plant, ferrous sulfate and sulfur are generated. Ferrous sulfate after the sulfur is separated therefrom is oxidized to ferric sulfate using biochemical treatment equipment with air being blown thereinto and is returned to the desulfurization plant.
In said desulfurization method, there are such problems that, microorganisms being used indirectly therefor, the system becomes complicated and the microorganisms used in the biochemical treatment equipment being iron-oxidizing bacteria to react at a normal temperature, the reaction velocity is slow and the plant becomes enormously large, etc.
In order to solve these problems, the inventors of the present invention have found, and filed a patent application (Japanese Pat. Appl. 93-84237) for, a method in which hydrogen sulfide-containing gas generated at a geothermal power plant is directly led to, and desulfurized in, a biochemical treatment tank using high temperature acidophilic sulfur-oxidizing bacteria, and culture liquid of the biochemical treatment tank acidified by sulfuric acid generated by desulfurization is added to hot water returning to a reduction well. With this method, an effective desulfurization treatment can be carried out and yet a blockade of the reduction well due to the pH value of the hot water returning to the reduction well becoming higher can be avoided.
A desulfurization method at a geothermal power plant according to said method is outlined below with reference to FIG. 9. In FIG. 9, hot water 201 taken out from a production well 1 is separated into steam 4 and hot water 202 by use of a flasher 3. The steam 4, being led to a steam turbine 5 for power generation and being condensed at a condenser 6 after used for power generation, is returned to a reduction well 7 together with the hot water 202. High density hydrogen sulfide-containing gas 8 which is not condensed at the condenser 6 is oxidized at a biochemical treatment tank 10 to low density hydrogen sulfide-containing gas 11 and is dispersed into the air.
The pH value of culture liquid of the biochemical treatment tank 10 is lowered with time together with the hydrogen sulfide being converted to sulfuric acid. Accompanying the sulfuric acid generation, the acidified culture liquid 48 is taken out from the biochemical treatment tank 10 and is injected into the reduction well 7 together with the hot water 202. The pH value of the hot water 202 being lowered thereby, a fear of a blockade of the reduction well 7 due to scale sticking is mitigated. On the other hand, new culture liquid 50, of the same amount as the acidified culture liquid 48 so taken out, is led into the biochemical treatment tank 10 from a culture liquid tank 49 and desulfurization is done continuously.
In said geothermal power plant desulfurization method, due to lowering of pH value of the culture liquid or due to increase of salt density by adding alkali for neutralization thereof, lowering of desulfurization rate at the biochemical treatment tank occurs with time, and for avoidance thereof, a large amount of culture liquid for exchange is required, thus inorganic nutritive salt to be used for culture liquid results in a high cost.